The recent advance in OA equipment is remarkable. In the field of printers, the wire dot system and thermal transfer system are being replaced by the ink jet system.
Printers of the ink jet system usually have an ink holder in the ink tank (or ink storing container) for stable ink holding. Highly improved printers require the ink holder to have not only good ink holding performance but also have many other capabilities. For example, the ink holder tends to dissolve in ink and the dissolved matter lowers the surface tension of ink, thereby causing defective printing. In order to address this problem, there has been proposed a new ink holder capable of stably supplying ink without denaturing ink, which is formed from a polyurethane foam of special polyol.
Even such an ink holder, however, is still liable to deterioration by ink in the ink tank. In addition, when the ink tank temporarily deforms and returns to its original shape, the ink holder does not follow such deformation due to plastic deformation, with the result that a shortcut for air occurs between the ink holder and the inside of the ink tank. The ink holder also undergoes plastic deformation when it is attached to the ink tank, it is vacuum-packed and unpacked, and it is piled up during storage for a long period of time, or when the ink tank is dropped. As mentioned above, once a shortcut occurs, air enters it to prevent smooth ink supply, thereby causing defective printing. This air entrapment readily causes defective printing in the case of modern highly sophisticated ink jet printers.
The present invention was completed in view of the foregoing. It is an object of the present invention to provide a polyurethane foam compound for ink holders, an ink holder which is obtained from the polyurethane foam compound by foaming, curing, and cross-linking, and a method for permitting the ink holder to remain in close contact with the inside of the ink tank for a long period of time. The ink holder mentioned above has an extremely low level of residual compressive strain and readily follows the deformation of the ink tank.
In order to achieve the above-mentioned object, the present inventors carried out a series of researches which led to the following finding. The polyurethane foam for ink holders, which is obtained from a polyurethane foam compound composed mainly of polyol and isocyanate, has very little residual compressive strain and readily follows the deformation of the ink tank while it is immersed in ink (an aqueous solution of hydrophilic organic solvent), if the polyurethane foam compound is incorporated with a cross-linking agent as an essential component.
The present inventors also found the following. The extreme reduction in residual compressive strain permits the polyurethane foam to retain stress over a long period of time. Consequently, the polyurethane foam as the ink holder remains in close contact with the inside of the ink tank for a long period of time after it has been placed in the ink tank. In addition, the ink holder keeps in close contact with the inside of the ink tank even when the ink tank is deformed by external pressure. This contributes to good printing performance over a long period of time.
The present inventors also found that the polyurethane foam obtained by foaming, curing, and cross-linking the above-mentioned polyurethane foam compound is suitable for use as the ink holder in modern highly sophisticated ink jet printers.
The present inventors also found the following. The ink holder, which is hardly affected by ink, remains in close contact with the inside of the ink tank over a long period of time, if it is formed from a polyurethane foam compound containing a cross-linking agent as an essential component as mentioned above. The present invention was completed on the basis of the above-mentioned findings.
Accordingly, the present invention is directed to a polyurethane foam compound for ink holders, an ink holder, and a method for permitting the ink holder to keep close contact.
(1) A polyurethane foam compound for ink holders which is characterized in being composed mainly of polyol and isocyanate and incorporated with a cross-linking agent.
(2) The polyurethane foam compound for ink holders as defined in (1) above, in which the cross-linking agent is a hydroxyl group-containing compound having an average molecular weight of 50 to 500 and 2 to 10 functional groups.
(3) The polyurethane foam compound ink holders as defined in (1) or (2) above, in which the cross-linking agent is one which has an OH value no less than 100.
(4) The polyurethane foam compound for ink holders as defined in any of (1) to (3) above, in which the cross-linking agent is added in an amount of 0.1 to 10 parts by weight for 100 parts by weight of polyol.
(5) The polyurethane foam for ink holders as defined in any one of (1) to (4) above, in which the polyol is at least one species selected from the group consisting of polyether polyol and polymer polyol grafted with styrene and/or acrylonitrile.
(6) An ink holder which is characterized in being obtained by foaming, curing, and cross-linking the polyurethane foam compound defined in any one of (1) to (5) above.
(7) The ink holder as defined in (6) above, which has a residual compressive strain equal to 10% or less which is measured in its released state after immersion under compression by 30% in an aqueous solution of hydrophilic organic solvent at 60° C. for 72 hours.
(8) The ink holder as defined in (6) or (7) above, which is intended for use in ink jet printers.
(9) A method for permitting an ink holder, which has been placed in an ink tank, to keep close contact with the inside of the ink tank, the method being characterized in that the ink holder is the one which is mentioned in any of (6) to (8) above.